This routine computes the gravitational acceleration in a
specified direction for a vector of points given by their
coordinates.

ARGUMENTS

iCoords,jCoords,kCoords: coordinates of the points where the
gravitational accelation is requested.
Each of these arrays should either be
of lenght numPoints (or more), in which
case its nth value is used for the nth
point; or else of dimension 1, in which
case the value is used for all points.
accelDir : The acceleration direction: allowed values are
IAXIS, JAXIS and IAXIS. These values are defined
in constants.h.
numPoints : Number of cells to update in accel()
accel : Array to receive results
blockID : The local identifier of the block to work on,
not applicable in pointmass gravity.
potentialIndex : optional, not applicable in pointmass gravity